The field of the invention is electrical motor controls, and more particularly, those controls which generate a heating current in motor windings during the time that a motor is at rest and disconnected from its power supply. As illustrated in Blair, U.S. Pat. No. 3,403,239, issued May 20, 1969, a motor heating control package may be connected to two conductors supplying power to a three-phase motor through a motor starter. As described in this patent the heating control circuit selectively couples a motor heating current to the motor after it has been disconnected from its power supply through the opening of the motor starter contacts. The heat generated by this current is said to keep the motor windings at 15.degree.-20.degree. C. above ambient to prevent condensation from forming on the windings. The mechanical features of this arrangement are not described in detail.
As illustrated in Burton et al., U.S. Pat. No. 3,896,353, a mechanical door interlock may be connected between an operating handle on the exterior of an equipment enclosure and a power supply disconnect switch, so that power to a motor starting contactor will be disconnected prior to the opening of an access door on the enclosure. This prior patent also discloses an electromechanical interlock for disabling the operating handle when the motor starting contactor is energized. An earlier motor control of the prior art provided an interlock between an operating handle and the motor starting contactor which required leverage to slide the contactor out of engagement with the plurality of stab connectors. The interlocking of the operating handle to the supply line disconnect switch reduced the mechanical load to be moved through the door interlock. Although this improved door interlock has been provided for motor starting controls, a comparable interlock feature has not been provided in motor heating controls, especially for a motor heating control that could be used to retrofit a motor control center where the customer desires the economy and convenience of retaining an installed motor starter.